We know that people born in winter months are at greater risk of neurological disorders, including serious conditions like bipolar disorder and schizophrenia. We're now starting to understand the reasons behind this phenomenon, and it's all about our biological clocks.

The idea that people and animals will act in certain ways based on what time of the year they were born is known as seasonal imprinting. This phenomeon has previously been directly observed in non-mammals, and there's lots of indirect evidence to support it in mammals, such as the increase in neurological disorders suffered by people born in winter months.

But new research by scientists at Vanderbilt has provided the first direct observation of seasonal imprinting in mammals, and it provides the first clear biological explanation for what's going on in humans. The researchers took two groups of baby mice and placed them in artificial light cycles, with one replicating winter light and the other replicating summer. The mice were raised from birth to weaning in these habitats, and then they were either kept there or moved to the opposite environment for another 28 days. Then they were put in constant darkness, and the researchers measured the mice's activity patterns.

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The winter-born mice showed a consistently slower daily activity period, suggesting they were generally more lethargic than their summer-born counterparts. These results also held up on a genetic level, where similar activity patterns were observed. The summer-born mice were also far more adaptable to changes in light cycles, as they easily adjusted their biological clocks to the new winter dusk, whereas the winter-born mice had a much more difficult time adjusting to summer.

Lead researcher Douglas McMahon explains how this ties into humans:

"Our biological clocks measure the day length and change our behavior according to the seasons. We were curious to see if light signals could shape the development of the biological clock. The mice raised in the winter cycle show an exaggerated response to a change in season that is strikingly similar to that of human patients suffering from seasonal affective disorder."

There's still a lot of stuff the researchers still have to work out, including whether these effects are permanent and when during the three-week period between birth and weaning mice actually develop their seasonal biological clock. Still, the basic finding is profound, as fellow researcher Chris Ciarleglio explains:

"What is particularly striking about our results is the fact that the imprinting affects both the animal's behavior and the cycling of the neurons in the master biological clock in their brains. We know from previous studies that light can affect the development of other parts of the brain, for example the visual system. Our work shows that this is also true for the biological clock."

Obviously, humans are more complex than mice, and this doesn't tell the whole story about why people, regardless of their birth month, develop conditions like seasonal affective disorder, much less more severe conditions like schizophrenia or bipolar disorder. But the findings in mice point to similar seasonal imprinting in humans, albeit in a subtler way. McMahon elaborates on this idea, also recognizing that all this might seem a bit unlikely:

"We know that the biological clock regulates mood in humans. If an imprinting mechanism similar to the one that we found in mice operates in humans, then it could not only have an effect on a number of behavioral disorders but also have a more general effect on personality. It's important to emphasize that, even though this sounds a bit like astrology, it is not: it's seasonal biology!"